Recent magnetic tapes for computer backup have been innovatively improved in high density recording capabilities and characteristics. It is predictable that large recording capacity tapes as several tens Terra bytes capacity a reel will be able to be made in the near future. For this high density recording, the more the magnetic tapes have data tracks, the narrower the widths of the data tracks and the gaps between two adjacent data tracks are to be shrunk down. The grain size of the magnetic material is reduced to obtain the finer recording pattern for such shrink down than before. The film base of the magnetic tape becomes thinner so that the longer tapes are obtained for a reel of the magnetic tape without increasing the reel diameters. According to increasing of the data tracks to realize high data recording capacity, the frequency to read or write the magnetic tape increases due to the serpentine read/write over the magnetic tape. It is quite important to consistently and correctly read the control signal recorded on the track. We call the signal obtained by a magnetic transducer which senses the control mark magnetically recorded on the track “a detected servo signal”, the track on which the control marks are magnetically recorded “a servo band” and the magnetically recording of a control mark onto a track “servo band writing” or “servo writing” in short. The control mark is physically a magnetic flux transition of the magnetic material which is coated on a magnetic tape. The scale down technology requires the small size magnetic transducers for both data recording onto the magnetic tape as magnetic flux transitions and data reproducing from such magnetic flux transition patterns recorded on the magnetic tape. The scale down technology also requires small dimension of the control marks recorded as the magnetic flux servo transition pattern as well as narrowing the data tracks and the servo bands. It is necessary to keep the high S/N in reading out the recorded data due to the small size magnetic flux transition patterns recorded on the narrowed tracks, especially for the servo bands. More specifically, the high signal resolution is necessary to precisely read out the control marks recorded as magnetic flux servo transition patterns, in other words the servo bands, by means of small size magnetic transducer. By this appropriate lateral positioning of the magnetic head to the servo bands, the correct reading of the data recorded in magnetic flux transition patterns on the data tracks which are adjacent to the servo bands becomes possible. For this purpose, the enhancement of the resolution of servo signal read becomes important to realize a correct tracking of the servo bands.
There are two schemes for the magnetic head lateral positioning control using the servo bands. One is an amplitude servo method by using the width of magnetic flux servo transition pattern recorded on the servo band which is read by the servo band read head and the other a timing-based servo method by using the signal timing in synchronous to servo burst in the servo bands. In the timing-based servo scheme, non-parallel pattern of magnetic flux servo transition stripes (called “servo stripes” hereinafter) is recorded on the servo band by the servo recording heads while the tape is fed through a servo writer. For this servo writer, magnetic recording heads fabricated by photolithography are used for the purpose of fine patterning of magnetic gap portion. The servo stripes are generated by the magnetic flux leaked at the magnetic gap of the magnetic head (see, for example, reference 1).
FIG. 8 shows the generating and reading the servo stripes in the conventional timing-based servo scheme.
When the current pulse is applied to the magnetic head, a servo stripe is generated as a magnetic flux servo transition by the magnetic flux leaked from the magnetic gap of the magnetic head. This magnetization can be done in a single pole due to the single polarity of the current pulse. On the other hand, no magnetic flux transition is generated when the current pulse is not applied to the magnetic head. The intensity of the current pulse PC to generate servo stripes is limited not to saturate a magnetic resistance (abbreviated as “MR”) element used for the magnetic head.
In the magnetic tape read out operation, the MR element used for the servo magnetic head senses the magnetic flux transition as the change of the resistance. The change of the resistance is measured by the voltage. The larger the change of the resistance, the better the S/N of the detected servo signal. The peak of the detected servo signal is in proportion to the amount of the magnetization of each servo stripe if the MR element has no saturation. Therefore, if the amount of the magnetization of the servo stripe is large then the peak voltage value of the detected servo signal RSL is large.
According to the requirement for large recording capacity tapes, the width of the servo band is shrunk down so that the magnetic flux to the transducer to sense the servo stripe becomes small. In the shrinking rule that is based on the principle of constant magnetic flux per area (which is magnetic flux density), when the thickness of the magnetic material decreases, then the magnetic flux becomes much less as in a rule of cubic in such shrink down of the scale. Therefore the detected servo signal RSS that is for the large recording capacity tapes decrease. On the other hand, the noise generated by the transducer is not scaled down in a rule of cubic. As the result, the S/N for RSS becomes worse and the stable signal revel corresponding to the servo magnetic head position onto the servo band is hardly obtained and the precise lateral position control of the magnetic head becomes difficult as far as using such detected servo signal RSS.
Reference 1:                Published Japanese Patent Application: Paragraph 0007 and the FIG. 5, JP, 2003-157634, A (2003)        
Reference 2:                U.S. Pat. No. 5,689,384        